Field of the Invention
Embodiments of the present invention relate to a technique for producing hydrogen in which hydrogen is generated by using high temperature steam electrolysis.
Related Art
Realization of a hydrogen energy society using hydrogen as an energy medium has attracted attention. As one of technologies for producing hydrogen, a high temperature steam electrolysis is widely known. The high temperature steam electrolysis is a method in which hydrogen and oxygen are generated by electrolyzing steam with a high temperature (normally, 500° C. or more).
The method has an advantage to reduce an amount of electricity required for electrolysis by electrolyzing steam under a high temperature environment as compared to electrolysis of water. Because of the characteristic as described above, an same amount of hydrogen can be obtained by using smaller electricity than that of the room temperature water electrolysis by about 30%. Thus, hydrogen can be produced at high energy efficiency.
Furthermore, since water is a raw material, it is possible to produce hydrogen without discharging carbon dioxide at all if electric power obtained by renewable energy that generates no carbon dioxide and a heat source that generates no carbon dioxide are used.
In the high temperature steam electrolysis, a hydrogen electrode and an oxygen electrode are provided on both sides of a solid oxide electrolyte to constitute an electrolytic cell. High temperature steam is supplied to the hydrogen electrode side, and an electrolytic voltage is applied to the both electrodes, so that the steam is decomposed to hydrogen and oxygen.
Conventionally, various hydrogen production apparatuses that safely and efficiently produce hydrogen by the high temperature steam electrolysis have been studied. A technology to generate hydrogen efficiently by the high temperature electrolysis with lower energy has been disclosed (for example, Japanese Patent Laid-Open Nos. 2002-348694, 2005-281716, 2013-49600).
In the above high temperature steam electrolysis, by performing an electrolytic reaction at a thermoneutral point at which an endothermic reaction in a process in which hydrogen and oxygen are generated from steam by the electrolytic reaction, and an exothermic reaction by electrical resistance of the electrolytic cell itself become almost equal to each other, it is possible to minimize energy input from outside, and produce hydrogen at high efficiency.
However, when the renewable energy is used as an electric power source for the electrolytic reaction, there is a problem that it is difficult to maintain an applied voltage to the electrolytic cell around the thermoneutral point since the electric power obtained by the renewable energy varies with time.
Also, when a time-varying electric power source is used, the electrolytic cell may be brought into a steam starvation state (steam starvation) or a surplus steam state due to a change in a steam rate used for the electrolytic reaction. Thus, there is a problem that it is difficult to perform an efficient hydrogen production operation according to the steam rate.